Periodontology (eBook)

1 Anatomy and Physiology

The periodontium (from the Greek terms περι, around, and ωδωνσ, tooth) denotes the soft and hard tissues that anchor the teeth to the bones of the jaws, provide interdental linkage of the teeth within the dental arch, and facilitate epithelial lining of the oral cavity in the region of the erupted tooth.1–3

It is a developmental, biological, and functional unit that is comprised of four different types of tissue:

Gingiva—that is, the marginal periodontium

Root cementum

Alveolar bone proper

Periodontal ligament

The gingiva, a keratinized soft tissue, surrounds the tooth at the cervical level together with parts of the alveolar process. The desmodontal fiber apparatus connects the various mineralized forms of root cementum, which are in some ways similar to bone tissue, and the alveolar bone proper, which is part of the alveolar process.

The majority of fibers consist of collagen. They insert partly in the inner cortical plate of the alveolar bone and partly in root cementum. The fibers of the periodontal ligament are functionally oriented. During and after tooth eruption they undergo continuous renewal and remodeling, which is mainly controlled by fibroblasts. Those fibers, which are anchored either in root cementum or in alveolar bone proper, are called Sharpey's fibers. Oxytalan fiber bundles, which run parallel to the tooth axis, may also be observed. Their function is largely unknown.

Development

The development of the periodontium is essentially linked to tooth development (Fig. 1.1). The number and shape of the teeth are strictly genetically determined. Tooth development is initiated by epithelial thickening of the ectodermal epithelial lining of the primitive oral cavity (stomodeum) between the fifth and sixth week of embryogenesis. This odontogenic epithelium is called the dental plate or dental lamina. Originating from the dental lamina, tooth development is controlled by a chain of cell–cell and cell–matrix interactions. Both ectodermal and ectomesenchymal cells reach increasingly higher degrees of differentiation and eventually develop into highly differentiated ameloblasts and odontoblasts which produce enamel matrix and predentin.

Crown Development

Between weeks 6 and 8 after ovulation, cells of the dental lamina proliferate in distinct regions (the later positions of deciduous teeth) into the mesenchymal tissue beneath. They induce a condensation of the ectomesenchymal tissue, which derives from the neural crest. This tissue is now called determinate dental mesenchyme.

During morphogenesis of the tooth germ, the tooth bud develops between the 8th and 12th weeks into the caplike enamel organ. The odontogenic mesenchyme divides into two cell lines:

The dental papilla, containing progenitor cells of odontoblasts and later the pulp.

The dental follicle, which surrounds the tooth germ and develops into the periodontium.

Cells of the enamel organ differentiate into four cytologically and functionally defined strata:

Outer enamel epithelium

Stratum reticulare

Stratum intermedium

Inner enamel epithelium

Finally, the bell stage of the tooth germ develops, which already gives an indication of the later shape of the crown. The enamel–dentin border is defined when odontoblasts and later ameloblasts differentiate and start, in the region of the later cusps and incisal edges of the teeth, secreting predentin and enamel matrix, respectively. The dental papilla and enamel organ are surrounded by the dental follicle, which demarcates the dental papilla from the surrounding mesenchyme.

During further odontogenesis the dental lamina of the deciduous molars proliferates distally and becomes committed to the development of the molars of the permanent dentition, which therefore belong to the first dentition.

The tooth germs of the permanent first molars arise between the 13th and 15th week of embryogenesis. The germs of succedaneous teeth arise between the 5th prenatal (central incisors) and 10th postnatal months (second premolars), and develop from an apically prolonged secondary dental lamina lingually and palatally to the deciduous germs.

Root Development

Root formation starts when dentin and enamel formation reaches the connection between inner and outer enamel epithelium, viz. the later cementoenamel junction. The final shape of the crown has now been determined.

Due to further proliferation of the enamel epithelium, an epithelial root sheath (Hertwig's root sheath) develops, which is located between the dental papilla and the dental follicle proper:

Apically, it bends inwards to form the epithelial diaphragm.

The double-layered epithelium (outer and inner stratum) is responsible for differentiation of root dentin-forming odontoblasts.

It thus represents the “mold” for the later tooth root.

Tooth eruption depends on the prolongation of the dentinal tube, while the diaphragm remains in the same location. Coronally, Hertwig's epithelial root sheath loses contact with the root surface. The sheath disintegrates into a loose mesh of epithelial strands, namely the epithelial rests of Malassez.

Cementogenesis

In contrast to the inner enamel epithelium of the enamel organ, the inner enamel epithelium of Hertwig's root sheath does not differentiate into enamel-producing ameloblasts:

Cell–cell interactions lead to differentiation of cells of the neighboring ectomesenchymal tissue of the dental papilla into odontoblasts, which start forming predentin.

Immediately afterwards an enamel-like material is deposited on the surface of predentin.

This material probably induces differentiation of cementoblasts derived from the dental follicle and mediates anchorage of the cementum to the dentin surface.

Following disintegration of the epithelial root sheath, cells of the dental follicle proper come into contact with the newly formed root surface (cell–matrix interaction). They then start formation of root cementum (Fig. 1.2). Cells and fiber bundles of the periodontal ligament and alveolar bone proper are also derived from cells of the dental follicle proper.

These traditional views of cementogenesis have been challenged5:

The presence of mesenchymal cells among disintegrated cells of Hertwig's epithelial root sheath is usually interpreted as a sign of cell migration from the dental follicle proper towards the root surface.

Alternatively, cells of the root sheet may have completed epithelial–mesenchymal transformation.

Thus, cementoblasts may originate from Hertwig's epithelial root sheath itself.

In multirooted teeth, shortly after dentin and enamel formation have commenced in the cusp region, two or three epithelial knots are formed in the region of the cervical loop of the enamel epithelium.

Tongues of epithelium proliferate across the dental papilla in a central direction.

While the size of the enamel organ increases, these epithelial tongues meet and fuse in the region of the future fornix of the furcation.

In this way, the future dentin floor of the furcation is created.

This means that the formation of the furcation is part of crown development.

The epithelial root sheath initiates the formation of tooth roots. It determines their shape. In case of multirooted teeth it divides into two or three branching tubes. The presence of enamel epithelium also explains frequent formation of enamel paraplasias in the furcation area, such as enamel projections, enamel tongues, and enamel pearls.

Epithelial tongues lie within the connective tissue of the dental papilla and exclude parts of ectomesenchymal tissue from the developing tooth germ. That is why cementum deposits (ridges and bulges) are frequently found in the region of fusing epithelial tongues.

Marginal Periodontium

The epithelial part of the gingiva is of ectodermal origin. Three epithelia can be distinguished:

Junctional epithelium

Oral sulcular epithelium

Oral gingival epithelium